Magnetic Resonance Imaging (MRI) apparatus is used in the medical field for diagnosis. Any object which is to be imaged by MRI apparatus must be placed within a spatially varying magnetic field and then subjected to a perturbation pulse of radio frequency (RF) radiation. An MRI imaging coil then detects the resulting nuclear magnetic resonance spectra which are combined to give cross-sectional images. When a patient is being "imaged" for medical diagnosis, a large electromagnetic coil which completely surrounds the patient is normally used. A separate imaging coil may be used on the skin surface of the patient to obtain improved resolution of the magnetic resonance image. In some MRI devices, one external coil is used for both excitation and reception, the two functions of the coil being interleaved in time.
Imaging tissues deep within the body with this system has two problems. First, the depth of the sample that can be imaged is approximately equal to the width of the imaging coil so the greater the depth to be imaged, the larger the imaging coil must be. Second, the signal-to-noise ratio will decrease with increased depth of the image so the resolution and image definition will also decrease with increased depth of the image. As a result, the resolution provided by conventional external imaging coils is not sufficient for controlling a therapy procedure. These problems can be overcome to a large degree by using an imaging coil that is placed within the body of the patient. With the imaging coil within the patient, it can be placed adjacent to the tissues of interest such that depth is no longer a limiting factor. The resolution and image definition for these tissues with an internal imaging coil will be much greater than that obtained with an external imaging coil.
A probe which provides the imaging from within the body is disclosed in U.S. Pat. No. 4,672,972 to Berke (the Berke patent). The Berke probe includes an imaging coil which may be tuned to an emission frequency of interest. The external processor for this probe may also excite the coil to radiate a localized perturbation field prior to obtaining the emission data of interest. The Berke probe is limited, however, to diagnostic functions and cannot perform any therapy functions.
Hyperthermia therapy can be accomplished using probes or applicators such as that disclosed in U.S. Pat. No. 4,823,812 to Eschel, et al. (the Eschel applicator). The Eschel applicator can be used for hyperthermia treatment by insertion into body cavities. The applicator includes a microwave antenna which generates radio frequency (RF) electromagnetic radiation for heating the body tissue. The Eschel applicator has no capability for imaging or locating the applicator within the patient's body.
One way to locate the applicator is through use of the catheter and probe disclosed in U.S. Pat. No. 4,813,429 to Eschel, et al. (the Eschel probe). The Eschel probe includes a diode for detection of the peak of the microwave field generated by the Eschel applicator. The Eschel applicator is then positioned until the peak is detected by the Eschel probe. The Eschel probe also contains thermocouples for measuring the heat produced by the Eschel applicator during the therapy. Therefore, to provide the hyperthermia therapy at the proper location requires use of both the Eschel applicator and the Eschel probe. No imaging of the surrounding tissues is achieved through use of these two devices.
A therapy device such as the Eschel applicator could also be used in conjunction with an alternative method of imaging for use in controlling the therapy procedure. Such alternatives include direct visual feedback (e.g., intraoperative cauterization), x-ray fluoroscopy, or ultrasound.
Accordingly, prior to the development of the present invention, no single device was capable of imaging the tissues for control of the therapy procedure, locating the therapy device in the correct location, and delivering the therapy. It is therefore an object of this invention to provide a probe which is capable of imaging tissues from within the body with sufficient resolution to control heat therapy, and which is capable of delivering the heat therapy. It is a further object to provide a probe with the capability for temperature monitoring of the heat therapy. It is an advantage of this invention that imaging and heat therapy can be performed using only the magnetic resonance imaging radio frequency source such that no independent RF source is required.